Laser produced plasma euv light source

ABSTRACT

An EUV light source is disclosed which may comprise a plurality of targets, e.g., tin droplets, and a system generating pre-pulses and main-pulses with the pre-pulses for irradiating targets to produce expanded targets. The system may further comprise a continuously pumped laser device generating the main pulses with the main pulses for irradiating expanded targets to produce a burst of EUV light pulses. The system may also have a controller varying at least one pre-pulse parameter during the burst of EUV light pulses. In addition, the EUV light source may also include an instrument measuring an intensity of at least one EUV light pulse within a burst of EUV light pulses and providing a feedback signal to the controller to vary at least one pre-pulse parameter during the burst of EUV light pulses to produce a burst of EUV pulses having a pre-selected dose.

This application is a Continuation of U.S. patent application Ser. No.12/928,313, filed on Dec. 7, 2010, and published on Apr. 7, 2011 as US2011-0079736-A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE,Attorney Docket Number 2006-0006-06; which is a Divisional of U.S.patent application Ser. No. 11/644,153 filed on Dec. 22, 2006, andissued on Apr. 19, 2011, as U.S. Pat. No. 7,928,416, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number 2006-0006-01,the contents of which are incorporated herein by reference.

The present application is related to U.S. patent application Ser. No.11/358,988, filed on Feb. 21, 2006, and published on Nov. 16, 2006, asU.S. 2006-0255298A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCEWITH PRE-PULSE, Attorney Docket Number 2005-0085-01; U.S. patentapplication Ser. No. 11/067,124 filed on Feb. 25, 2005, now U.S. Pat.No. 7,405,416, issued on Jul. 29, 2008, entitled METHOD AND APPARATUSFOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number2004-0008-01; U.S. patent application Ser. No. 11/174,443 filed on Jun.29, 2005, now U.S. Pat. No. 7,372,056, issued on May 13, 2008, entitledLPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney DocketNumber 2005-0003-01; U.S. patent application Ser. No. 11/358,983 filedon Feb. 21, 2006, now U.S. Pat. No. 7,378,673, issued on May 27, 2008,entitled TARGET MATERIAL DISPENSER FOR EUV LIGHT SOURCE, Attorney DocketNumber 2005-0102-01; U.S. patent application Ser. No. 11/358,992 filedon Feb. 21, 2006, now U.S. Pat. No. 7,598,509, issued on Oct. 6, 2009,entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number2005-0081-01; U.S. Provisional Patent Application Ser. No. 60/775,442filed on Feb. 21, 2006, entitled EXTREME ULTRAVIOLET LIGHT SOURCE,Attorney Docket Number 2006-0010-01; U.S. patent application Ser. No.11/174,299 filed on Jun. 29, 2005, now U.S. Pat. No. 7,439,530, issuedon Oct. 21, 2008, and entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM,Attorney Docket Number 2005-0044-01; U.S. patent application Ser. No.11/406,216 filed on Apr. 17, 2006, now U.S. Pat. No. 7,465,946, issuedon Dec. 16, 2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE,Attorney Docket Number 2006-0003-01; and U.S. patent application Ser.No. 11/580,414 filed on Oct. 13, 2006, now U.S. Pat. No. 7,491,954,issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUVLIGHT SOURCE, Attorney Docket Number 2006-0025-01, the entire contentsof each of which are hereby incorporated by reference herein.

FIELD

The present disclosure relates to extreme ultraviolet (“EUV”) lightsources which provide EUV light from a plasma that is created from atarget material and collected and directed to a focus for utilizationoutside of the EUV light source chamber, e.g., for semiconductorintegrated circuit manufacturing photolithography e.g., at wavelengthsof around 50 nm and below.

BACKGROUND

Extreme ultraviolet light, e.g., electromagnetic radiation havingwavelengths of around 50 nm or less (also sometimes referred to as softx-rays), and including light at a wavelength of about 13.5 nm, can beused in photolithography processes to produce extremely small featuresin substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has an element, e.g.,xenon, lithium or tin, with one or more emission line in the EUV range.In one such method, often termed laser produced plasma (“LPP”) therequired plasma can be produced by irradiating a target material, suchas a droplet, stream or cluster of material having the requiredline-emitting element, with a laser beam.

Heretofore, LPP systems have been disclosed in which each droplet isirradiated by a separate laser pulse to form a plasma from each droplet.Also, systems have been disclosed in which each droplet is sequentiallyilluminated by more than one light pulses. In some cases, each dropletmay be exposed to a so-called “pre-pulse” and a so-called “main pulse”,however, it is to be appreciated that more than one pre-pulse may beused and more than one main pulse may be used and that the functions ofthe pre-pulse and main pulse may overlap to some extent. Typically, thepre-pulse(s) may function to expand the material and thereby increasethe amount of material which interacts with the main pulse and themain-pulse may function to convert most or all of the material into aplasma and thereby produce an EUV light emission. However, it is to beappreciated that the functions of the pre-pulse and main pulse mayoverlap to some extent, e.g., the pre-pulse(s) may generate some plasma,etc. The increased material/pulse interaction may be due a largercross-section of material exposed to the pulse, a greater penetration ofthe pulse into the material due to the material's decreased density, orboth. Another benefit of pre-pulsing is that it may expand the target tothe size of the focused pulse, allowing all of the pulse to participate.This may be especially beneficial if relatively small droplets are usedas targets and the irradiating light cannot be focused to the size ofthe small droplet.

In addition to the above described techniques, U.S. Pat. No. 6,855,943(hereinafter the '943 patent) which issued to Shields on Feb. 15, 2005and is entitled “DROPLET TARGET DELIVERY METHOD FOR HIGH PULSE-RATELASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE” discloses a technique inwhich only some of the droplets in a droplet stream, e.g., every thirddroplet, is irradiated to produce a pulsed EUV light output. Asdisclosed in the '943 patent, the nonparticipating droplets (so-calledbuffer droplets) advantageously shield the next participating dropletfrom the effects of the plasma generated at the irradiation site.Unfortunately, in some cases, these buffer droplets may reflect lightback into the laser causing self-lasing, which among other things, canreduce the effectiveness of the laser's gain media in producing highenergy pulses. This may be especially true for high gain (e.g.,G=1000-10,000) infra-red lasers, e.g., CO₂ lasers, which tend toself-lase rather easily.

A typically photolithography scanner exposes a portion of a moving waferto a so-called “burst” of light pulses. In many cases, the pulse energyvaries from pulse-to-pulse within the burst, and the accumulated energyof the burst, referred to generally as “dose”, is typically prescribedand must be controlled within a relatively small range. In addition todose, some lithography operations also prescribe limits on thepulse-to-pulse energy variation within a burst. This is sometimesreferred to as pulse-to-pulse energy stability. The lasers that are usedin deep ultraviolet photolithography (DUV), e.g., excimer lasers at awavelength of, e.g., 100-300 nm, typically have the ability tosubstantially vary pulse energy within a burst of pulses, and in somecases, on a pulse-to-pulse basis. This can be achieved, for example, byaltering the discharge voltage used to create each pulse. Thisflexibility, however, is not available for all types of lasers. In somelaser architectures, a continuously pumped active media may be used togenerate laser pulses. For example, pulses may be generated in acontinuously pumped oscillator using, for example, Q-switch or cavitydump mode, and then amplified by one or more continuously pumped poweramplifiers. As used herein, the term continuously pumped laser devices(CW laser devices) refers to a laser device having a continuously pumpedactive media, e.g., continuously pumped oscillator and/or continuouslypumped amplifier. Unlike the pulse—pumped laser devices described above,e.g., excimer discharge laser devices, the CW laser devices do not havethe ability to quickly vary their discharge voltage, and as aconsequence, are generally incapable of substantially varying theiroutput pulse energy within a burst of pulses by altering their dischargevoltage.

As indicated above, one technique to produce EUV light involvesirradiating a target material droplet with one or more pre-pulse(s)followed by a main pulse. In this regard, CO₂ lasers, and in particular,continuously pumped CO₂ laser devices may present certain advantages asa drive laser producing “main” pulses in an LPP process. This may beespecially true for certain targets, e.g., tin. For example, oneadvantage may include the ability to produce a relatively highconversion efficiency e.g., the ratio of output EUV in-band power todrive laser input power.

Along these lines, U.S. Pat. No. 6,973,164 which issued to Hartlove etal. on Dec. 6, 2005 and is entitled “LASER-PRODUCED PLASMA EUV LIGHTSOURCE WITH PRE-PULSE ENHANCEMENT” discloses that a variation in timedelay between pre-pulse and main pulse in a Nd:YAG with Xenon targetsLPP system results in a variation of output EUV intensity for timedelays shorter than 160 ns.

With the above in mind, Applicants disclose a laser produced plasma EUVlight source, and corresponding methods of use.

SUMMARY

In a first aspect of an embodiment of the present application, an EUVlight source may comprise a plurality of targets, e.g., tin droplets,and a system generating pre-pulses and main-pulses with the pre-pulsesfor irradiating targets to produce expanded targets. The system mayfurther comprise a continuously pumped laser device generating the mainpulses with the main pulses for irradiating expanded targets to producea burst of EUV light pulses. The system may also have a controllervarying at least one pre-pulse parameter during the burst of EUV lightpulses. In addition, the EUV light source may also include an instrumentmeasuring an intensity of at least a portion of one EUV light pulsewithin a burst of EUV light pulses and providing a feedback signal tothe controller to vary at least one pre-pulse parameter during the burstof EUV light pulses.

For this aspect, the pre-pulse parameter may be a delay time between apre-pulse and the corresponding main pulse and/or the pre-pulseparameter may be the pulse energy of the pre-pulse. In one embodiment ofthis aspect, the continuously pumped laser device may comprise a CO₂laser device and in a particular embodiment the pre-pulses may passthrough the continuously pumped laser device.

In another aspect, a method for generating EUV light may comprise thesteps/acts of irradiating at least one target material droplet(s) withan initial light pulse to create irradiated target material, exposingirradiated target material to a subsequent light pulse to generate EUVlight, measuring an intensity of the EUV light generated, using themeasurement to calculate an energy magnitude for a subsequent lightpulse, and thereafter irradiating a target material droplet with a lightpulse having the calculated energy magnitude. In one implementation, thesubsequent light pulse may be generated using a continuously pumpedlaser device and in a particular implementation, the continuously pumpedlaser device may comprise a CO₂ laser device and the target material maycomprise tin.

For another aspect of an embodiment, an EUV light source may comprise aplurality of target material droplets traveling along a path, e.g., asubstantially linear path, and a system generating an initial pulse anda subsequent pulse. The initial pulse is provided for irradiating adroplet on the path to generate irradiated target material exiting thepath, and the subsequent pulse is focused to a location for interactionwith the irradiated target material. For this aspect, the location isdistanced from the path and the subsequent pulse is provided forexposing the irradiated target material to generate EUV light.

In one implementation of this aspect, only every other droplet travelingalong the path is irradiated by a respective pre-pulse. In someembodiments, the initial pulse and the subsequent pulse may travel alonga common beam path. In particular, the first and subsequent pulse may befocused using a common optic and the initial pulse and the subsequentpulse may have differing beam divergences prior to focusing. The lightsource may further comprise one or more saturable absorbers positionedto absorb photons reflected from the target material. Alternatively, orin addition thereto, the system may comprise an amplifier amplifying thesubsequent pulse and the light source may further comprise an opticalisolator positioned along a beam path between the target material andamplifier. In a particular embodiment, the light beam within theamplifier may have a primary polarization direction and the opticalisolator may comprise a phase retarding optic, e.g., phase retardingmirror and a polarizer.

In another aspect of an embodiment of the present application, an EUVlight source may comprise a plurality of targets and a system generatinglaser pulses, the laser pulses for irradiating targets to produce aburst of EUV light pulses. For this aspect, the EUV light source mayfurther comprise an instrument measuring an intensity of at least aportion of one EUV light pulse within a burst of EUV light pulses andproviding a feedback signal indicative thereof, and a shutter responsiveto the feedback signal to trim a laser pulse, the trimmed laser pulsegenerating an EUV light pulse during the burst of EUV light pulses. Forthis aspect, the portion of the EUV pulse that is measured may be atemporal portion, a spatial portion, or a temporal and spatial portion.

In one implementation of this aspect, the portion of the EUV pulse thatis measured may be a temporal portion and the EUV pulse generating themeasured intensity may be produced by the trimmed laser pulse. Forexample, the system may comprise an optical oscillator outputting laserpulses. Each output laser pulse may have a pulse duration, t, and thetravel time, T, for light traversing an optical path extending from theoptical oscillator output through the target and to an intensitymeasurement site may be less than the pulse duration exiting theoscillator, T<t. In a particular embodiment, each output laser pulse mayhave a pulse duration, t>T+T_(s), where T_(s) may define a system timeincluding, for example, a detector response time, a processor responsetime, a driver response time, a switch response time and a signalpropagation time, in one arrangement, the detector, switch and processormay be positioned closely together to minimize T_(s). Furthermore, thesystem may comprise at least one optical amplifier receiving laser lightfrom the optical oscillator along an optical path and the shutter may bepositioned on the optical path between the optical amplifier and theoptical oscillator.

In another implementation of this aspect, the EUV pulse measured toproduce the feedback signal may be generated by a corresponding laserpulse and the feedback signal produced therefrom may be used to trim asubsequent laser pulse. In either of these implementations, the shuttermay comprise an electro-optic switch. For example, the system generatinglaser pulses may comprise an optical oscillator defining a primarypolarization direction for light exiting the oscillator and the shuttermay comprise an electro-optic switch having an electro-optic cell, e.g.Pockels or Kerr cell, and a polarizer.

In one implementation, the system generating laser pulses may comprisean optical oscillator and at least one optical amplifier and in a firstparticular implementation the optical oscillator may be a cavity dumpedoscillator and in a second particular implementation the opticaloscillator may be a transverse electrical-discharge oscillator having apulsed electrical-discharge power source. For any of theseimplementations, the amplifier may comprise a continuously pumped laserdevice.

One implementation of the EUV light source employs a chamber in whichthe laser pulses irradiate the targets. For this implementation, theintensity measuring instrument may be positioned inside the chamber,outside the chamber or in some cases, multiple instruments may beemployed, e.g. one inside and one outside the chamber. For example, anintensity measuring instrument may be positioned in a lithographyscanner that is coupled to the chamber and/or an intensity measuringinstrument may be positioned at a location optically downstream of alithography scanner that is coupled to the chamber.

In another aspect of an embodiment of the present application, an EUVlight source may comprise a plurality of targets and a system generatinglaser pulses which comprises a transverse, electrical-dischargeoscillator having a pulsed electrical-discharge power source. The laserpulses may be provided for irradiating targets to produce a burst of EUVlight pulses and the EUV light source may further comprise an instrumentmeasuring an intensity of at least a portion of one EUV light pulsewithin a burst of EUV light pulses, with the instrument providing afeedback signal indicative thereof. A controller responsive to thefeedback signal may be provided to establish a discharge voltage for asubsequent laser pulse, the subsequent laser pulse generating an EUVlight pulse during the burst of EUV light pulses. In one particularimplementation, the transverse electrical-discharge oscillator maycomprise a CO₂ oscillator.

In still another aspect of an embodiment of the present application, anEUV light source may comprise a target material and a system having atleast one optic establishing a beam path with the target material. Forthis aspect, the system may provide an initial light pulse for aninitial photon interaction with the target material, the initialinteraction generating a reflected pulse passing back and forth alongthe beam path between the optic and target material and producing aplurality of subsequent photon interactions with the target material.Each of the subsequent interactions may produce an EUV emission and thesystem may have an optical gain medium positioned along the beam path.The EUV light source may further comprise an instrument measuring an EUVemission intensity and providing a feedback signal indicative thereof;and a shutter responsive to the feedback signal to selectively limit thenumber of subsequent photon interactions generated by the initial lightpulse.

In a particular embodiment of this aspect, the system may comprise anoptical oscillator coupled with the beam path to provide the initiallight pulse. In one implementation the instrument may measure an EUVemission intensity from a subsequent interaction corresponding to aninitial pulse and generate a feedback signal for limiting the number ofsubsequent photon interactions generated by the same initial lightpulse. In another implementation the instrument may measure an EUVemission intensity from a subsequent interaction corresponding to aninitial pulse and generate a feedback signal for limiting the number ofsubsequent photon interactions generated by a different, e.g. later,initial light pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for alaser-produced plasma EUV light source according to an aspect of anembodiment;

FIG. 2A shows a schematic view of portions of an EUV light sourceillustrating the control EUV output intensity by varying one or morepre-pulse parameters;

FIG. 28 shows another embodiment in which the source producing a trainof pre-pulses and the source producing a train of main pulses may sharea common pulse amplifier;

FIG. 2C shows another embodiment in which the pre-pulses and main pulsesmay travel along different beam paths to reach the target volume;

FIG. 2D shows another arrangement which may be employed to control EUVoutput dose and/or pulse to pulse stability;

FIG. 2E1 shows another arrangement which may be employed to control EUVoutput dose and/or pulse to pulse stability;

FIG. 2E2 shows another arrangement which may be employed to control EUVoutput dose and/or pulse to pulse stability;

FIG. 2F shows a plot of EUV output intensity versus time having a firstEUV pulse corresponding to an initial LPP interaction and subsequent EUVpulses corresponding to reflected LPP pulses;

FIG. 2G shows a plot of EUV output intensity versus time as in FIG. 2Fbut after a shutter has eliminated some of the “subsequent EUV pulses”to control dose and/or pulse to pulse stability;

FIG. 3A shows a schematic view of a plurality of target materialdroplets that are traveling along a common path immediately prior toirradiation by a pre-pulse;

FIG. 3B shows a schematic view of a plurality of target materialdroplets that are traveling along a common path immediately followingirradiation of a droplet by a pre-pulse;

FIG. 4 shows a schematic view of an embodiment in which the pre-pulseand main pulse travel along parallel beam paths and are focused using acommon optic to focal spots at different locations;

FIG. 4A shows a schematic view illustrating the focusing of a pre-pulsefor the arrangement shown in FIG. 4; and

FIG. 4B shows a schematic view illustrating the focusing of a main pulsefor the arrangement shown in FIG. 4.

DETAILED DESCRIPTION

With initial reference to FIG. 1 there is shown a schematic view of anexemplary EUV light source, e.g., a laser produced plasma EUV lightsource 20 according to one aspect of an embodiment. As shown in FIG. 1,and described in further detail below, the LPP light source 20 mayinclude a system 22 for generating light pulses and delivering the lightpulses into a chamber 26. As detailed below, the light pulses may travelalong one or more beam paths from the system 22 and into the chamber 26to illuminate one or more targets at an irradiation region 28.

As further shown in FIG. 1, the EUV light source 20 may also include atarget material delivery system 24, e.g., delivering droplets of atarget material into the interior of a chamber 26 to the irradiationregion 28 where the droplets will interact with one or more lightpulses, e.g., one or more pre-pulses and thereafter one or more mainpulses, to ultimately produce a plasma and generate an EUV emission. Thetarget material may include, but is not necessarily limited to, amaterial that includes tin, lithium, xenon or combinations thereof. TheEUV emitting element, e.g., tin, lithium, xenon, etc., may be in theform of liquid droplets and/or solid particles contained within liquiddroplets or any other form which delivers the EUV emitting element tothe target volume in discrete amounts. For example, the element tin maybe used as pure tin, as a tin compound, e.g., SnBr₄, SnBr₂, SnH₄, as atin alloy, e.g., tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, or a combination thereof. Depending on thematerial used, the target material may be presented to the irradiationregion 28 at various temperatures including room temperature or nearroom temperature (e.g., tin alloys, SnBr₄) at an elevated temperature,(e.g., pure tin) or at temperatures below room temperature, (e.g.,SnH₄), and in some cases, can be relatively volatile, e.g., SnBr₄. Moredetails concerning the use of these materials in an LPP EUV source isprovided in U.S. patent application Ser. No. 11/406,216 filed on Apr.17, 2006, now U.S. Pat. No. 7,465,946, issued on Dec. 16, 2008, entitledALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number2006-0003-01, the contents of which has been previously incorporated byreference herein.

Continuing with FIG. 1, the EUV light source 20 may also include acollector 30, e.g., a reflector, e.g., in the form of a truncatedellipsoid, e.g., a graded multi-layer mirror having alternating layersof Molybdenum and Silicon, with an aperture to allow the light pulsesgenerated by the system 22 to pass through and reach the irradiationregion 28. The collector 30 may be, e.g., an ellipsoidal mirror that hasa first focus within or near the irradiation region 28 and a secondfocus at a so-called intermediate point 40 (also called the intermediatefocus 40) where the EUV light may be output from the EUV light source 20and input to a device utilizing EUV light, e.g., an integrated circuitlithography tool (not shown).

The EUV light source 20 may also include an EUV controller 60, which mayalso include a firing control system 65 for triggering one or more lampsand/or laser devices in the system 22 to thereby generate light pulsesfor delivery into the chamber 26. The EUV light source 20 may alsoinclude a droplet position detection system which may include one ormore droplet imagers 70 that provide an output indicative of theposition of one or more droplets, e.g., relative to the irradiationregion 28. The imager(s) 70 may provide this output to a dropletposition detection feedback system 62, which can, e.g., compute adroplet position and trajectory, from which a droplet error can becomputed, e.g., on a droplet by droplet basis or on average. The dropleterror may then be provided as an input to the controller 60, which can,for example, provide a position, direction and/or timing correctionsignal to the system 22 to control a source timing circuit and/or tocontrol a beam position and shaping system, e.g., to change the locationand/or focal power of the light pulses being delivered to theirradiation region 28 in the chamber 26.

As further shown in FIG. 1, the EUV light source 20 may include adroplet delivery control system 90, operable in response to a signal(which in some implementations may include the droplet error describedabove, or some quantity derived therefrom) from the controller 60, toe.g., modify the release point of the target material from a dropletdelivery mechanism 92 to correct for errors in the droplets arriving atthe desired irradiation region 28.

For the EUV light source 20, the droplet delivery mechanism 92 mayinclude, for example, a droplet dispenser creating either 1) one or morestreams of droplets exiting the dispenser or 2) one or more continuousstreams which exit the dispenser and subsequently break into dropletsdue to surface tension. In either case, droplets may be generated anddelivered to the irradiation region 28 such that one or more dropletsmay simultaneously reside in the irradiation region 28 allowing one ormore droplets to be simultaneously irradiated by an initial pulse, e.g.,pre-pulse to form an expanded target suitable for exposure to one ormore subsequent laser pulse(s), e.g., main pulse(s), to generate an EUVemission. In one embodiment, a multi-orifice dispenser may be used tocreate a “showerhead-type” effect. In general, for the EUV light source20, the droplet dispenser may be modulating or non-modulating and mayinclude one or several orifice(s) through which target material ispassed to create one or more droplet streams. More details regarding thedispensers described above and their relative advantages may be found inU.S. patent application Ser. No. 11/358,988, filed on Feb. 21, 2006, andpublished on Nov. 16, 2006, as U.S. 2006/0255298A1, entitled LASERPRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number2005-0085-01; U.S. patent application Ser. No. 11/067,124 filed on Feb.25, 2005, now U.S. Pat. No. 7,405,416, issued on Jul. 29, 2008, entitledMETHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, AttorneyDocket Number 2004-0008-01 and U.S. patent application Ser. No.11/174,443 filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issuedon May 13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERYSYSTEM, Attorney Docket Number 2005-0003-01, the contents of each ofwhich were previously incorporated by reference.

FIG. 1 also shows, schematically, that the EUV light source 20 mayinclude one or more EUV metrology instruments 94 for measuring variousproperties of the EUV light generated by the source 20. These propertiesmay include, for example, intensity (e.g., total intensity or intensitywithin a particular spectral band), spectral bandwidth, polarization,etc. For the EUV light source 20, the instrument(s) 94 may be configuredto operate while the downstream tool, e.g., photolithography scanner, ison-line, e.g., by sampling a portion of the EUV output, e.g., using apickoff mirror or sampling “uncollected” EUV light, and/or may operatewhile the downstream tool, e.g., photolithography scanner, is off-line,for example, by measuring the entire EUV output of the EUV light source20.

FIG. 2A shows in more detail an aspect of an embodiment in which EUVoutput intensity may be controlled within a burst of pulses, and in somecases on a pulse-to-pulse basis, to provide a predetermined energy dose,e.g., energy dose within a predetermined range. As shown there, thesystem 22′ may include two separate devices 300, 302 that are used togenerate the pre-pulse(s) and main pulse(s), respectively. FIG. 2A alsoshows that a beam combiner 306 may be employed to combine pulses fromthe devices 300, 302 along a common beam path 308. Device 300 may be alamp, e.g., producing incoherent light, or a laser and typically isselected to produce output pulses having controllable energy within aburst, and in some cases on a pulse-to-pulse basis. Suitable lasers foruse as the device 300 may include pulsed gas discharge lasers such asexcimer, CO₂, etc, pulsed solid state lasers, e.g., disk shaped Nd:YAG,etc.

Light device 302 is typically a laser and may be a different type oflaser than used for device 300. Suitable lasers for use as the device302 may include a pulsed laser device, e.g., a pulsed gas discharge CO₂laser device producing radiation at 9.3 μm or 10.6 μm, e.g., with DC orRF excitation, operating at relatively high power, e.g., 10 kW and highpulse repetition rate, e.g., 24 kHz or more. In one particularimplementation, a continuously pumped CO₂ laser device may be used forthe device 302. For example, a suitable CO₂ laser device having anoscillator and three amplifiers (O-PA1-PA2-PA3 configuration) isdisclosed in U.S. patent application Ser. No. 11/174,299, filed on Jun.29, 2005, now U.S. Pat. No. 7,439,530, issued on Oct. 21, 2008, andentitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney DocketNumber 2005-0044-01, the entire contents of which have been previouslyincorporated by reference herein. Depending on the application, othertypes of lasers may also be suitable, e.g., an excimer or molecularfluorine laser operating at high power and high pulse repetition rate.Examples include, a solid state laser, e.g., having a fiber or diskshaped active media, a MOPA configured excimer laser system, e.g., asshown in U.S. Pat. Nos. 6,625,191, 6,549,551, and 6,567,450, an excimerlaser having one or more chambers, e.g., an oscillator chamber and oneor more amplifying chambers (with the amplifying chambers in parallel orin series), a master oscillator/power oscillator (MOPO) arrangement, apower oscillator/power amplifier (POPA) arrangement, or a solid statelaser that seeds one or more excimer or molecular fluorine amplifier oroscillator chambers, may be suitable. Other designs are possible.

As further illustrated by FIG. 2A, the device 300 generates a train ofpre-pulses 310 a,b that are delivered to an irradiation region 28 a. Atthe target volume, each pre-pulse may irradiate at least one droplet(s)to produce an expanded target. Also, device 302 generates a train ofmain pulses 312 a,b with each main pulse for irradiating a respectiveexpanded target at or near the irradiation region 28 a to produce an EUVlight output which can then be measured by the instrument 94 (shown inFIG. 1).

FIG. 2A also illustrates an example of a system architecture for varyinga pre-pulse parameter during the train of pre-pulses in response to asignal from the measurement instrument 94, e.g., intensity detector. Asshown in FIG. 2A, the controller 60 may receive a signal, e.g., afeedback signal, e.g., a signal indicative of EUV output intensity, fromthe instrument 94 and in turn, communicate with firing control system 65to independently trigger one or both of the sources 300, 302 and/orcontrol the discharge voltage of the device 300. Although the controller60 and firing control system 65 are shown as discrete elements, it is tobe appreciated that the controller 60 and firing control system 65 maybe integrated into a common unit and/or may share one or moreprocessors, memory boards, etc.

With the above-described arrangement, one or more pre-pulse parametersmay be varied such that the EUV output intensity of a subsequent, e.g.,next, EUV output pulse may be controlled. For example, the pre-pulseparameter may be a delay time, Δt, between a pre-pulse 310 a and acorresponding main pulse 312 a. This may be accomplished by controllingthe triggering times for the devices 300, 302. Alternatively, or inaddition thereto, the pulse energy of the pre-pulse may be varied tocontrol the EUV output intensity. In one implementation, a processor inthe controller 60 may have access to memory holding a dataset, e.g.electronic lookup table, (which may include empirically derived and/orcalculated data) having a tabulated correspondence between one or morepre-pulse parameters and EUV output intensity. Thus, in oneimplementation, an algorithm may be processed to track a running averageof the EUV output intensity during a burst. Based on the running averageor some similar parameter, the algorithm may further be processed tocalculate a desired EUV output intensity for a subsequent, e.g., next,EUV output pulse, selected to obtain a desired dose. The algorithm maythen query the dataset to retrieve one or more pre-pulse parameterscorresponding to the desired EUV output intensity and initiate thetransmission of a signal indicative of the pre-pulse parameter(s) to thefiring control system 65. The firing control system 65 may thenimplement the pre-pulse parameter(s) by controlling the trigger timesand/or discharge voltages for the device 300 such that a subsequent,e.g., next, pre-pulse/main pulse droplet irradiation is performed withthe selected pre-pulse parameter.

FIG. 2B shows another embodiment having a system 22″ in which the sourceproducing a train of pre-pulses and the source producing a train of mainpulses may share a common pulse amplifier 314. For this setup, the mainpulses may be generated using a four chamber O-PA1-PA2-PA3 architectureas shown and designated 316 (oscillator), 318 (PA1), 320 (PA2), 314(PA3). Pulses generated by the laser device 300′ may pass through the PA314 for amplification, prior to delivery to the irradiation region 28 b,as shown. Although three amplifiers are shown in FIG. 2B, it is to beappreciated that more than three and as few as one amplifier may bespecified for the system 22″, For the arrangement shown in FIG. 2B, thecontroller 60′ may receive a signal, e.g., a feedback signal, e.g., asignal indicative of EUV output intensity, from the measuring instrument94′ and in turn, communicate with firing control system 65′ toindependently trigger the device 300′ and/or the oscillator 316 and/orcontrol the discharge voltage of the device 300′. In this manner, apre-pulse parameter, e.g., a delay time between a pre-pulse and acorresponding main pulse and/or the pulse energy of the pre-pulse, maybe varied to control the EUV output intensity, e.g., on a pulse-to-pulsebasis, and thereby control EUV output dose and/or pulse stability.

FIG. 2C illustrates that pulses from the light sources 300″, 302″ maytravel along different beam paths 322, 324 to reach the irradiationregion 28 c. For the arrangement shown in FIG. 2C, controller 60″ mayreceive a signal, e.g., a feedback signal, e.g., a signal indicative ofEUV output intensity, from the measurement instrument 94″ and in turn,communicate with firing control system 65″ to independently trigger oneor more of the devices 300″, 302″ and/or control the discharge voltageof the device 300″. In this manner, a pre-pulse parameter, e.g., a delaytime between a pre-pulse and a corresponding main pulse and/or the pulseenergy of the pre-pulse, may be varied to control the EUV outputintensity, e.g., on a pulse-to-pulse basis, and thereby control EUVoutput dose and/or pulse stability.

FIG. 2D shows another arrangement which may be employed to control EUVoutput dose and/or pulse stability. As shown there, the system 22′″ mayinclude a device 326, e.g. oscillator generating laser pulses, e.g.,main pulses, with each pulse having a pulse duration, and an amplifier327 having one or more amplifying chambers. Note: for some embodimentsthe device 326 may also provide pre-pulses or a separate device (notshown) may be included to provide pre-pulses, As shown, the system 22′″may also include a shutter 328 operable to alter, e.g., trim, a pulsesuch that only a temporal portion of the pulse is delivered to theirradiation region 28 d to illuminate the target material.

FIG. 2D also illustrates that controller 60′″ may receive a signal,e.g., a feedback signal, e.g., a signal indicative of EUV outputintensity, from the measuring instrument 94′″ and in turn, communicatewith the shutter 328. Also shown, the controller 60′″ may communicatewith the firing control system 65′″ to trigger and/or control thedischarge voltage of the device 326. For the EUV light source shown inFIG. 2D, the shutter 328 (shown schematically) may include anelectro-optic switch, e.g. having a time response in the nanosecondrange, e.g., Pockel's or Kerr cell, and a polarizer. For example, thedevice 326, e.g., CO₂ laser device, may employ polarizer(s) and/orBrewster's windows such that light exiting the device 326 has a primarypolarization direction. With this arrangement, the shutter may includean electro-optic switch and a polarizer having a transmission axisaligned orthogonal to the primary polarization direction defined by thedevice 326. Thus, when the switch is energized, light is able to passfrom the device 326 to the irradiation region 28 d. On the other hand,when the switch is de-energized, light exiting the device 326 is rotatedand is absorbed and/or reflected (away from the beam path leading to theirradiation region 28 d) by the polarizer.

In one setup, the device 326 may be configured to produce and outputpulses having a pulse duration, t, that is longer than the travel time,T, for light traversing the optical path extending from the output ofthe device 326, through the target, and to the intensity measurementsite. With this arrangement, EUV light generated by the leading portionof the laser pulse may be measured and the measurement used to trim thetrailing portion of the same laser pulse. In some cases, a pulsestretcher (not shown) may be used to provide pulses having t>T. In aparticular embodiment, each output laser pulse may have a pulseduration, t>T+T_(s), where T_(s) may define a system time including, forexample, a detector response time, a processor response time, a driverresponse time, a switch response time and a signal propagation time. Inone arrangement, the measuring instrument 94″, shutter 328 andcontroller 60′″ may be positioned closely together to minimize T_(s). Itis to be appreciated that this same technique (i.e. measuring EUVproduced by a leading portion of a laser pulse and trimming the trailingportion of the same laser pulse) may also be used on pulses whose pulseduration, t, is shorter than the travel time, T, for light traversingthe optical path extending from the output of the device 326, throughthe target, and to the intensity measurement site. For example, thisfunctionality may be obtained by moving the shutter 328 to a position onthe optical path that is relatively close to the irradiation site.However, the arrangement shown in FIG. 2D in which the shutter 328 ispositioned upstream of the amplifier may be beneficial in someapplications due to the relatively low laser light intensity at theshutter 328.

The arrangement shown in FIG. 2D may also be configured to control EUVoutput dose and/or pulse stability by measuring an EUV light pulseparameter, e.g., intensity, for EUV light produced by a first laserpulse and using the measurement to thereafter trim a subsequent laserpulse. For this arrangement, the device 326 may be, but is notnecessarily limited to, a cavity dumped oscillator or a transverseelectric-discharge oscillator.

FIG. 2E1 shows an EUV light source having an arrangement which may beemployed to control EUV output dose and/or pulse to pulse stability. Asshown there, the light source may include a system having a device 326′,e.g., oscillator generating laser pulses, e.g., main and/or pre-pulses,e.g., a CO₂ master oscillator, and an amplifier having one or moreamplifying chambers 327 a, 327 b, 327 c. Note: for some embodiments, aseparate device (not shown) may be included to provide pre-pulses. Forthe EUV light source, the device 326′ may provide an initial light pulsewhich is directed by beam combiner 330 into chamber 26′ where the lightpulse produces an initial photon interaction with the target material332, e.g. droplet produced by a droplet generator 334.

FIG. 2E1 further shows that the system may include on optic 336 and apair of turning mirrors 338 a,b which together with the target material332 establish a beam path 340. For the system shown, the optic 336 maybe, for example, a reflecting mirror or corner reflector. The EUV lightsource may also include a shutter 328′ switchable between a first stateallowing light to travel between the amplifier 327 c and optic 336 and asecond state preventing light travel between the amplifier 327 c andoptic 336. For the EUV light source, the shutter 328′ may include anelectro-optic switch, e.g., having a time response in the nanosecondrange, e.g., Pockel's or Kerr cell, and a polarizer. For example, one ormore of the amplifiers 327 a-c and/or device 326′, may employpolarizer(s) and/or Brewster's windows such that light traveling betweenturning mirror 338 a and shutter 328′ has a primary polarizationdirection. With this arrangement, the shutter 328′ may include anelectro-optic switch and a polarizer having a transmission axis alignedorthogonal to the primary polarization direction defined by the device326′. Thus, when the switch is energized, light is able to pass betweenthe amplifier 327 c and optic 336. On the other hand, when the switch isde-energized, the light will have a polarization orthogonal to thepolarizer transmission axis and will be reflected away from the beampath leading to the target material 332 by the polarizer. Alternatively,the shutter 328′ may include an acousto-optic switch, e.g., an acoustooptic cell with RF amplitude control, that is switchable between a firststate allowing light to travel between the amplifier 327 c and optic 336and a second state preventing light travel between the amplifier 327 cand optic 336.

FIG. 2E1 further shows that the EUV light source may also include acontroller 60 a having a processor and one or more drivers. As shown,the controller 60 a may receive a signal, e.g., a feedback signal, e.g.,a signal indicative of an EUV output intensity, from a measuringinstrument 94 a and in turn, communicate with the shutter 328′.

FIG. 2E2 shows an EUV light source arrangement (having one or morecomponents in common with the arrangement shown in FIG. 2E1) which maybe employed to control EUV output dose and/or pulse to pulse stability.As shown, an EUV light source may include a device 326″, e.g.,oscillator generating laser pulses (main pulses and/or pre-pulses asdescribed above) an amplifier having one or more amplifying chambers 327a′, 327 b′, 327 c′ (as described above) a pair of turning mirrors 338a′,b′ which together with the target material 332′ and at least oneoptic in the device 326′ (e.g. output coupler and/or rear reflector)establish a beam path 340′. For the EUV light source, the device 326″may provide an initial light pulse which is directed through theamplifying chambers 327 a′, 327 b′, 327 c′ (which may or may not amplifythe intial pulse) into chamber 26″ where the light pulse produces aninitial photon interaction with the target material 332′, e.g., dropletproduced by a droplet generator 334′.

FIG. 2E2 further shows that the EUV light source may include a shutter328″ (e.g., electro-optic or acouto-optic as described above) switchablebetween a first state allowing light to travel between the amplifier 327c′ and device 326′ and a second state preventing light travel betweenthe amplifier 327 c′ and device 326′. FIG. 2E2 also shows that the EUVlight source may include a controller 60 a′ having a processor and oneor more drivers. As shown, the system controller 60 a′ may receive asignal, e.g., a feedback signal, e.g., a signal indicative of an EUVoutput intensity, from a measuring instrument 94 a′ and in turn,communicate with the shutter 328″. In the operation of the EUV lightsource shown in FIG. 2E2, the device 326″ may provide an initial lightpulse, e.g., laser pulse, which is directed along beam path 340′ intochamber 26″ where the light pulse produces an initial photon interactionwith the target material 332′. This light pulse may be of sufficientintensity to produce a substantial EUV emission, (e.g., main pulse) ormay constitute a pre-pulse, as described above. In either case, theinitial interaction may generate a reflected pulse passing back andforth along beam path 340′ between one or more optics in the device 326″and the target material 332′ producing a plurality of subsequent photoninteractions with the target material 332′. As described above, thenumber of subsequent interactions may be controlled by the shutter 328″to control dose and/or pulse set to pulse set stability.

The operation of the EUV light source shown in FIGS. 2E1 and 2E2 canbest be appreciated with cross reference to FIGS. 2F and 2G. Asindicated above, the device 326′, 326″ may provide an initial lightpulse, e.g., laser pulse, produces an initial photon interaction withthe target material 332, 332′. This light pulse may be of sufficientintensity to produce a substantial EUV emission, (e.g., main pulse) ormay constitute a pre-pulse, as described above. In either case, theinitial interaction may generate a reflected pulse passing back andforth along beam path 340, 340′ between the optic 336 (FIG. 2E1) or anoptic in the device 326″ (FIG. 2E2) and target material 332 and producea plurality of subsequent photon interactions with the target material332. This is illustrated in FIG. 2F which shows an EUV output pulse set342 having a first EUV pulse 342 a corresponding to the initialinteraction (from a main pulse) and a plurality of EUV pulses 342b,c,d,e,f corresponding to the subsequent photon interactions whichresult from the reflected pulse traveling back and forth along the beampath 340, 340′. Significant EUV output pulses due to the subsequentphoton interactions may, at some point, terminate, as shown, e.g., aslosses exceed gain along the beam path 340, 340′. The period betweenpulses in a pulse set will depend on the length of the beam path 340,340′, and by way of example, a beam path with an optical path length ofabout 56 meters between target material 332 and optic 336 will result ina period between peaks of about 340 ns. FIG. 2F shows that the pulse set342 will be followed by another EUV output pulse set 344 having an EUVoutput pulse 344 a initiated when a “new” light pulse from the device326′, 326″ interacts with a “new” droplet of target material. As shown,the EUV output pulse set 344 may also include a plurality of EUV pulses344 b,c,d,e,f corresponding to the subsequent photon interactions whichresult from the reflected pulse traveling back and forth along the beampath 340, 340′. In general, the time period between pulse 342 a and 344a is dependent on the pulse repetition rate of the device 326, and maybe, for example, about 13.9 uS for a pulse repetition rate of about 72kHz.

For the EUV pulse sets 342, 344 shown in FIG. 2F and described above,correspond to an operation of the EUV light source shown in FIGS. 2E1,2E2 in which the shutter 328′, 328″, remains open. Thus, the number ofinteractions due to reflections has not been limited by the shutter328′, 328″. For comparison, FIG. 2G shows an EUV output pulse set 346resulting from the operation of the EUV light source shown in FIGS. 2E1,2E2 in a manner such that the number of interactions due to reflectionshave been selectively limited by the shutter 328′, 328″ to control doseand/or pulse set to pulse set stability. In particular, the pulse set346 having pulses 346 a,b,c,d has been limited to three interactions dueto reflections (pulses 346 b,c,d) and the pulse set 348 having pulses348 a,b,c,d,e has been limited to four interactions due to reflections(pulses 348 b,c,d,e).

In more detail, EUV intensities corresponding to one or more of thepulses 346 a,b,c,d may be measured by instrument 94 a, 94 a′ and used togenerate a signal, e.g. feedback signal which is communicated tocontroller 60 a, 60 a′. The controller 60 a, 60 a′ may, in response tothe received signal, determine the number of interactions due toreflection required to meet a dose and/or pulse set to pulse setstability specification. For this determination, the controller may, insome cases, use intensity data from previous pulse sets. Afterprocessing, the controller 60 a, 60 a′ may then signal the shutter 328′,328″ using an appropriate driver to de-energize the shutter 328′, 328″at the appropriate time to limit further interactions due to reflection.As described above, the controller 60 a, 60 a′ may use measurementswithin a pulse set to limit interactions due to reflection in that samepulse set. In one implementation, the controller 60 a, 60 a′ mayintegrate the intensity generating a pulse energy for each pulse andthen accumulate the pulse energy for a given shot until the accumulatedpulse energy reaches a pre-selected level. At this point, the controller60 a, 60 a′ may de-energize the shutter and stop additional generationof the EUV by subsequent reflections.

For the arrangement shown in FIGS. 2E1, 2E2, the shutter 328′, 328″ andcontroller 60 a, 60 a′ may be located in the vicinity of the instrument94 a, 94 a′ to minimize the signal transfer time. Typically, for thecase where an instrument measurement within a pulse set is used to limitthe next interaction, the EUV light source may be configured such thatthe system response time (which is equal to the sum of time of signaltransfer, data integration and analysis and optical switch delay time)is less than the temporal separation between the reflected pulses,which, as described above, may be about 340 ns. In an alternativeimplementation, the controller 60 a, 60 a′ may use measurements within apulse set to limit interactions due to reflection in a different, e.g.,subsequent, pulse set. FIG. 3A illustrates a plurality of targetmaterial droplets that are traveling along a common path 350, which forthe illustration is a substantially linear path. The stream may, forexample be created by a droplet dispenser as described above and befalling under the influence of gravity. Droplet paths that are curvedmay also be used, for example, the droplets may be charged and thendeflected, the droplets may follow a curve trajectory under theinfluence of gravity, etc. FIG. 3A illustrates that some of the dropletson the common path, e.g., droplet 352 a may be irradiated by an initialpulse 354, e.g., pre-pulse that is focused to a focal spot at or nearthe common path 350. FIG. 3A also shows that some droplets, e.g.,droplets 352 b,c may be allowed to pass through the irradiation zone,and thus, do not participate in the production of plasma and EUV light.Instead, for the arrangement shown in FIG. 3A, only every third dropletis irradiated with the non-participating droplets (so-called bufferdroplets) shielding the next participating droplet from the plasma.

FIG. 3B shows the droplets after a short period of time, e.g., 1-100 μshas elapsed after the droplet 352 a shown in FIG. 3A is irradiated by aninitial pulse, e.g., pre-pulse. As shown, the effect of the initialpulse is to spatially expand the droplet creating an expanded volume 356that, due to the initial pulse irradiation, has exited the path 350.FIGS. 3A and 3B further show that a subsequent pulse 358, e.g., mainpulse, may be delivered to a location 360 that is distanced from thepath 350 to expose the expanded volume 356 and produce an EUV output. Inparticular, as shown, the subsequent pulse 358 may be focused to a focalspot at or near the location 360 such that a substantial intensity isnot presented at locations along the linear path 350. With thisarrangement, reflections to the device producing the subsequent pulsefrom the buffer droplets are not created, and as a consequence, thenumber of reflected photons available to cause self lasing in the deviceare reduced. For the arrangement shown in FIG. 3B, a substantialintensity is also avoided at locations along the linear path 350 duringthe delivery of a subsequent pulse, e.g., main pulse. As shown, this maybe accomplished by directing the initial pulse and subsequent pulsealong different beam-paths.

As illustrated in FIGS. 3A and 3B, the initial pulse 354 and subsequentpulse 358 may be focused to respective focal spots in the droplet 352 aand in the expanded volume 356. However, it is to be appreciated thatthe main pulse focal spot need not necessarily lie within the targetvolume 356 and the pre-pulse focal spot need not necessarily lie withinthe droplet 352 a. Stated another way, the initial pulse traveling alongthe beam path shown may be unfocused, or alternatively, may be focusedto a focal spot ahead of or behind the droplet 352 a. Similarly, thesubsequent pulse traveling along the beam path shown may be unfocused,or alternatively, may be focused to a focal spot ahead of or behind theexpanded volume 356.

FIG. 4 shows an embodiment in which the initial pulse, e.g., pre-pulseand the subsequent pulse, e.g., main pulse may travel alongsubstantially parallel beam paths 400, 401 and may be focused using acommon optic 402. The spacing of the parallel beam paths may compensatefor movement of material in the expanding volume. The focusing of aninitial pulse relative to the linear path 350′ for the arrangement shownin FIG. 4 is illustrated in FIG. 4A and the focusing of a subsequentpulse is shown in FIG. 4B. As described below, for this embodiment, asubstantial intensity may be avoided at locations along the linear path350′ during delivery of the subsequent pulse (reducing dropletreflections and/or laser droplet coupling and the associatedself-lasing). Stated another way, the subsequent pulse may be in asubstantially de-focused condition at locations along the linear path350′ as illustrated in FIG. 4B. As shown, this may be performed whilestill focusing the initial pulse 354′ at or near the droplet 352 a′(FIG. 4A) and focusing the subsequent pulse 358′ at or near the expandedvolume 356′ (FIG. 4B). This may be accomplished, for example, by usingan initial pulse beam and a subsequent pulse beam that have differentbeam divergences prior to focusing. This difference in divergence mayarise because the initial pulse and subsequent pulse may be initiated bydifferent sources 300′″, 302′″ and/or one or more optical arrangementsmay be provided to alter the beam divergence of the initial pulse beamand/or subsequent pulse beam.

As further shown in FIG. 4, the EUV light source 20′ may include anoptical isolator 404 positioned along the beam path 400 to reduce thenumber of reflected photons, e.g., photons reflected from droplets, fromreaching the light sources 300′″ and or 302′″ where the photons maycause self lasing and reduce the efficiency of the laser gain media toproduce controllable, repeatable pulses. For the light source 20′, theoptical isolator 404 may include one or more saturable absorbers. Forexample, a chamber having opposed windows and filled with a saturableabsorber material may be provided. The type of saturable absorbermaterial may be selected for the wavelength of interest, e.g., SF₆ gasfor light at a wavelength of 10.6 μm. Alternatively, or in addition tothe saturable absorber(s), the optical isolator 404 may include aswitchable shutter or a so-called isolator box (details not shown).

In one construction of an isolator box, the source 302′″, e.g., CO₂laser device may employ polarizer(s) and/or Brewster's windows such thatlight exiting the source 302′″ has a primary polarization direction.With this arrangement, the isolator box may include, for example, aphase retarder mirror which rotates back reflected light ninety degreesout of the primary polarization direction and an isolator mirror whichabsorbs light with the rotated polarization. For example, a suitableunit for use with CO₂ lasers may be obtained from Kugler GmbH,Heiligenberger Str. 100, 88682 Salem Germany under the trade nameQueller and/or “isolator box”. Typically, the isolator box functions toallow light to flow from the source 302′″ to the droplet substantiallyunimpeded while allowing only about one percent of back-reflected lightto leak through the isolator box and reach the source 302′″.

It will be understood by those skilled in the art that the aspects ofembodiments of the present invention disclosed above are intended to bepreferred embodiments only and not to limit the disclosure of thepresent invention(s) in any way and particularly not to a specificpreferred embodiment alone. Many changes and modification can be made tothe disclosed aspects of embodiments of the disclosed invention(s) thatwill be understood and appreciated by those skilled in the art. Theappended claims are intended in scope and meaning to cover not only thedisclosed aspects of embodiments of the present invention(s) but alsosuch equivalents and other modifications and changes that would beapparent to those skilled in the art. While the particular aspects ofembodiment(s) described and illustrated in this patent application inthe detail required to satisfy 35 U.S.C. §112 are fully capable ofattaining any above-described purposes for, problems to be solved by orany other reasons for or objects of the aspects of an embodiment(s)above described, it is to be understood by those skilled in the art thatit is the presently described aspects of the described embodiment(s) ofthe present invention are merely exemplary, illustrative andrepresentative of the subject matter which is broadly contemplated bythe present invention. The scope of the presently described and claimedaspects of embodiments fully encompasses other embodiments which may nowbe or may become obvious to those skilled in the art based on theteachings of the Specification. The scope of the present invention issolely and completely limited by only the appended claims and nothingbeyond the recitations of the appended claims. Reference to an elementin such claims in the singular is not intended to mean nor shall it meanin interpreting such claim element “one and only one” unless explicitlyso stated, but rather “one or more”. All structural and functionalequivalents to any of the elements of the above-described aspects of anembodiment(s) that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Any term usedin the Specification and/or in the claims and expressly given a meaningin the Specification and/or claims in the present application shall havethat meaning, regardless of any dictionary or other commonly usedmeaning for such a term. It is not intended or necessary for a device ormethod discussed in the Specification as any aspect of an embodiment toaddress each and every problem sought to be solved by the aspects ofembodiments disclosed in this application, for it to be encompassed bythe present claims. No element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element in the appended claims is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited as a “step” instead of an“act”.

1. EUV light source comprising: a target material delivery system fordelivering a plurality of targets; a system generating laser pulses onan output beam path, the laser pulses for irradiating targets to producea burst of EUV light pulses; an instrument measuring an intensity of atleast a portion of one EUV light pulse within a burst of EUV lightpulses and providing a feedback signal indicative thereof; and a shutterpositioned on the output beam path responsive to the feedback signal totrim a laser pulse, the trimmed laser pulse generating an EUV lightpulse during the burst of EUV light pulses.
 2. A light source as recitedin claim 1 wherein the portion of the EUV pulse measured is selectedfrom the group of portions consisting of a temporal portion, a spatialportion and a temporal and spatial portion.
 3. A light source as recitedin claim 1 wherein the portion of the EUV pulse measured is a temporalportion and the EUV pulse generating the measured intensity is producedby the trimmed laser pulse.
 4. A light source as recited in claim 3wherein said system comprises an optical oscillator outputting laserpulses with each output laser pulse having a pulse duration, t, andwherein the travel time, T, for light traversing an optical pathextending from the optical oscillator output to the target and from thetarget to an intensity measurement site is less than the pulse durationexiting the oscillator, T<t.
 5. A light source as recited in claim 4wherein the system further comprises at least one optical amplifierreceiving laser light from the optical oscillator along an optical pathand the shutter is positioned on the optical path between the opticalamplifier and the optical oscillator.
 6. A light source as recited inclaim 1 wherein the EUV pulse measured to produce the feedback signal isgenerated by a corresponding laser pulse and the feedback signalproduced is used to trim a subsequent laser pulse.
 7. A light source asrecited in claim 1 wherein said shutter comprises an electro-opticswitch.
 8. A light source as recited in claim 1 wherein said systemgenerating laser pulses comprises an optical oscillator and at least oneoptical amplifier.
 9. A light source as recited in claim 8 wherein theoptical oscillator is a cavity dumped oscillator.
 10. A light source asrecited in claim 8 wherein the optical oscillator is a transverseelectrical-discharge oscillator having a pulsed power source.
 11. An EUVlight source as recited in claim 8 wherein the amplifier comprises acontinuously pumped laser device.
 12. An EUV light source as recited inclaim 1 wherein said system generating laser pulses comprises an opticaloscillator defining a primary polarization direction and the shuttercomprises an electro-optic switch having an electro-optic cell and apolarizer.
 13. An EUV light source as recited in claim 1 furthercomprising a chamber, the laser pulses irradiating targets in thechamber, and wherein the intensity measuring instrument is positionedoutside the chamber.
 14. An EUV light source as recited in claim 13wherein the intensity measuring instrument is positioned in alithography scanner coupled to the chamber.
 15. An EUV light source asrecited in claim 13 wherein the intensity measuring instrument ispositioned at a location optically downstream of a lithography scannercoupled to the chamber.
 16. An EUV light source comprising: a targetmaterial delivery system for delivering a plurality of targets; a systemgenerating laser pulses on an output beam path, the laser pulses forirradiating targets to produce EUV light pulses; a means for measuringan intensity of at least a portion of one EUV light pulse and providinga feedback signal indicative thereof; and a laser pulse trimming meanspositioned on the output beam path responsive to the feedback signal totrim a laser pulse.
 17. A light source as recited in claim 16 whereinthe portion of the EUV pulse measured is selected from the group ofportions consisting of a temporal portion, a spatial portion and atemporal and spatial portion.
 18. A light source as recited in claim 16wherein the portion of the EUV pulse measured is a temporal portion andthe EUV pulse generating the measured intensity is produced by thetrimmed laser pulse.
 19. A method of producing EUV light comprising thesteps of: providing a plurality of targets; generating laser pulses onan output beam path, the laser pulses for irradiating targets to produceEUV light pulses; measuring an intensity of at least a portion of oneEUV light pulse and providing a feedback signal indicative thereof; andtrimming, using a shutter, a laser pulse on the output beam path inresponse to the feedback signal.
 20. The method as recited in claim 19wherein the portion of the EUV pulse measured in said measuring step isa temporal portion and the EUV pulse generating the measured intensityis produced by the trimmed laser pulse.